How Many Koi Can I Have in a Pond?

This seemingly simple question is actually very complicated. Based on size and design, each ecosystem must have its own distinct fish load, water flow rates, oxygen levels, filtration system, the number of aquatic plants, the types of plants involved, feeding schedules, food types, the frequency of water changes and ratios… and the list goes on.

We generally offer two rules of thumb regarding the number of koi in a pond. First, average water conditions will allow 1 inch of fish per 10 gallons of water (or 100 inches for a 1,000-gallon pond). Second, due to pheromones and toxin control, we want to stay at or under four koi per 1,000 gallons of water to promote a healthy living environment. These recommendations are a starting point and would be the all-encompassing answer to the question if it were a simple question. But since we love our koi, we need to have a deeper consideration for their needs to secure their healthy ecosystem.

Going Big? Decide Early.

This is why one of the first conversations between the owner and professional must focus on the end desire and how many koi the pond owner would like to have. This conversation must happen before you start the design so that the future koi and the pond owner will have the environment they need to maximize their potential — the enjoyment of your work.

I have too many customers with small water gardens that start the fish-load conversation with comments like, “I wish the pond were bigger.” As it turns out, these customers’ expectations of their pond are usually never met, because they want more fish than their pond was built to handle. To truly provide these types of customers with years of pleasure at their pond, you must not limit their numbers of fish by building too small or using inferior equipment systems.

Koi need space to grow, swim, exercise and explore. We not only want to look at pond square footage, but also consider water depth, as this will control the temperature during hot summer months. The total volume is also key, because as we increase the mass of water, we create a safer buffer to disperse toxins and nitrates, minimize rapid temperature changes in the pond and give the koi more space to cruise around and build muscle. I personally do not like seeing koi in ponds smaller than 2,000 gallons, because we really cannot provide both the depth and the horizontal space they need to be healthy when they grow into adults larger than 20 inches.

Position your client for success by building a koi pond rather than a water garden, and use the equipment their koi family members deserve.

This is not to say that you cannot create a very healthy ecosystem for a few koi in a 2,000-gallon pond with proper water flow, filtration and aeration — you absolutely can.

Water Flow & Aeration

Water flow and aeration accomplish the same main goal — keeping the water cool and rich in oxygen. With more oxygen in the water, the ecosystem will be healthier. The fish can swim and breathe more easily, and beneficial bacteria like Nitrosomonas and Nitrobacter can reproduce and convert toxins.

This 7-foot-deep koi pond is not only crystal clear, but also has optimal water stability and health from the oversized bog filter. Size, volume, filtration, aeration and plants create the perfect ecosystem.

By rule of thumb, we like to circulate koi ponds once an hour. However, we can increase oxygen levels and create a healthier ecosystem if we increase the flow or add an aerator to the pond, thereby increasing the capacity of fish the pond can handle safely. Your aerator is a supplement, but it should not be overlooked, especially in deeper ponds that need circulation on the bottom. Air pumps are easy to install quickly and can instantly increase a pond’s fish-load capacity, in turn fostering a very healthy ecosystem.

Filter, Filter, Filter

Your filter choice is the backbone of the koi pond ecosystem. The filter serves as a breeding ground and housing area for beneficial bacteria to do their magic, so the larger or more efficient your filter, the better job the bacteria is going to do with filtering. Oversizing the filter for your pond can permit a higher fish load, because a larger filter system can handle more fish.

Along with size, we also consider function. Filters that backflush frequently are getting cleaned more often and thus are ready for more action. These filters are preferred over filters you clean monthly or quarterly, because biowaste is removed more quickly. Avoid installing filters that are hard to clean, like putting rocks and filter pads inside a waterfall box, for example. Customers need to be able to clean the filter at least monthly, so you have to make it easy for them.

Another piece of filtration that needs to be installed is a properly sized UV clarifier. With the ultraviolet light’s ability to drastically reduce algae reproduction, the filter is inherently more efficient in breaking down fish waste and avoids getting filled with algae.

Nature’s Filter

Yet another filter option is adding plants. A bog filter that incorporates plants and a gravel bed in the filtration process will provide nitrate control and added oxygen through aquatic photosynthesis. The presence of plants is greatly encouraged in all ponds as a supplemental filtering agent and ecosystem protector.

Design with the koi in mind, and then create the beauty for your client. The specs, equipment choice and health of the koi must all come first.

Waterlilies provide shade, which cools the water table, while large-root bog plants such as iris, reeds and cannas provide constant toxin
reduction. Possibly most important are oxygenating plants like Anacharis and Cabomba that can stabilize the water table and absorb enormous amounts of nitrates while providing high amounts of oxygen back into the water throughout the day. One bog plant or oxygenating plant per 10 square feet of surface area and up to 50 percent waterlily coverage is ideal. These are easy to add to the pond and will make a huge impact on both the ecosystem and the visual softness, which your client will definitely notice.

The Necessary Extras

Fish load is truly all about water quality, which is good news, because you have control of water quality as you design. The pump, filter and aeration systems are key, but you cannot stop there.

Install skimmers and bottom drains to increase the quality and purity of the water table. If we allow sludge to build on the pond floor, we lose some of that quality. Designing the pond so that all the fish waste exits quickly, gets trapped in the filter and then is released weekly will greatly improve the water quality and thereby increase the fish-load capacity for your customer.

So…Final Answer?

Knowing that koi lovers will always want more koi, it is of greatest importance to build a pond with the proper equipment that will allow the owner to extend the fish load as desired. Ideally, the pond’s maximum fish load should always be greater than the number of fish the client has or wants at the moment.

Our koi are our babies. As social creatures, you can pet them, hand-feed them and even train them to eat from a bottle, so their happiness is very much dependent on us.

In the end, you determine how many koi your client can have with your design and build. You are the professional, and the client needs your expertise in the design as well as your advice regarding maintenance and water quality. You above all others can explain the system’s strengths and limitations. For the pond owner’s future enjoyment, make sure to explain the importance of weekly backflushing and partial water changes. Also help them with food choice and the proper way to stock their pond a little at a time.

So maybe the question doesn’t have an exact answer, but you have control of the answer. Position your client for success by building a koi pond rather than a water garden, and use the equipment their koi family members deserve. They will thank you for years to come for your attention to detail and shared passion.

However, when I talk to people (consumers) on the phone, they already have an overstocked pond. It is generally vastly overstocked and I doubt they could have anticipated the “koi-buying-sickness” that strikes new pond owners. I love the article. I think it is a great way to instill the responsibility on the homeowner right up front when installing a new pond as to stocking this pond. Gives them something to think about. Hopefully their conscience will kick in before the fish start dying and maybe send them back to the initial installer for upgrades, if needed.

Here is the link to John Russell’s article (of the same title): http://www.russellwatergardens.com/calculations/how-many-fish-can-i-have-in-my-pond/
However, I loved Shane’s “slant” on this in his article in Pond Trade. It is a dialogue that has to start before the pond digging has begun or the hand shake at the beginning of the deal. Put the responsibility of the pond (inhabitants and stocking) on the shoulders, squarely, of the pond owners. Let their conscience be their guide. It will build trust in the relationship with their contractor! Very good article and simply written.

I have a 1800-2000 gallone pond where we HAD eight nice looking koi. Turns out that things may have gotten a little out of control as I was late in discussing family planning with the group. We now have eight PLUS 3 newly discovered “babies”. What do you suggest I do? They obviously did not listen to me about proper birth control!

Ron, This is not an uncommon occurrence and you have several options. If you want to keep all 11 fish, I would recommend adding aeration and go to a routine 10%/week water change or something to that effect to help control the nitrates and water quality issues that may arise from overstocking. I would also look at your filtration systems and if it is one of the high quality Koi Filters, you probably will be alright with your family size…but if it is not, I would recommend taking the opportunity to invest for the future health of your pond and upgrade. You can feel free to call us at Water Garden Gems for more specific guidance or another expert Koi specialty location if you would like. If you are looking to share some of your family, check into local Koi Clubs or Koi Rescue services and see about finding a new great home for some. Again, if you want to detail out your system beyond the raw number of fish, give us a call and we are happy to help you.

I have a pond that calculators would suggest is 2100 gallons. It runs between 24” and 36” deep.
Currently I have six Koi in this pond ranging between 16” and 22” with the total being 120 inches.
I am running an Aquaforce 2700 pump and a picked up a Laguna MaxFlo 950. I have two Laguna Pressure Filters – (1400 and a 2100 models). The bottom is granite river rock and granite bolders.

I am planning to continue with a waterfall (3’ high and about 7’ long) in the spring that will split into two streams spilling into the 24” deep pools at the shallow end of the pond.

I have installed 6X6 columns for a pergola at the perimeter of the pond. The cedar beams crossing over top of the pond support a monorail allowing me to run a 9’ X14’ ShadeFX canopy over the water. So I have the ability to provide considerable summer shade and sky-view-protection from Heron’s in the summer. My pond has other shade elements (a large granite slab overhangs the water in the deep end and there is a 3’ diameter mill stone fountain in the deep end, as well.

Even after head losses for the waterfalls, I suspect I will be capable of circulating 1 ½ times my pond volume every hour.

Other things I could bring into the equation:
I have two aerators, if required
I have a Pondovac 3 for occasional maintenance of the bottom

My Questions:
Full Time Residents
I would like to add two more larger 20” Koi (Sanke and Asagi) to my pond – increasing the fish length to 160 inches.

Temporary Guests
I also have the idea that I would like to buy small (6”?) high-quality koi in the spring; allow them to grow over the summer; and sell them off (not necessarily for profit) in the fall. If I were to attempt this, how many of these smaller fish could I include, given the compensating factors I have talked about above?

Doug,
Thanks for your question. I would first say YES to using the aeration you have available. The more oxygen the better. Adding a couple nice Koi to the mix should be perfectly fine, but I would recommend frequent backflushes of your pressure filters as you will at that point be maxing out your system so I would expect the filters to fill up rather quickly. I personally would probably backflush them weekly, of course paying attention to the amount of waste you are removing and adjust accordingly. I would also make sure that you have good circulation along the bottom of your pond to minimize the waste settling into the pond floor and getting stuck in your aggregate. Definitely vacuum the pond…at least twice a year…at your major season changes. Food choice is important as well and you will want to feed a high quality food that leaves minimal waste such as HaiFeng or some of the other well respected Koi Food Brands.
As for your temporary guests, I think with due diligence on your part keeping the ecosystem clean, there would not be a problem. Keep in mind though that some of the key elements required in “grow-out” situations such as you are describing are water quality, O2 levels, and feeding (a lot of feeding…2-3 times a day) and so a pond that is already near it’s fish load is not your optimal environment. My only other recommendation here is know that your pond is going to be full…so you will not want any of those temporary guests to change their status to permanent citizens.

I am building a round concrete pond 7 feet deep half set in earth half above and walled around. It will have an 18 foot diameter and hold some 12,834 gallons of water turning over 7000 units of water an hour from pumps. I am working on 40 fish but can be corrected by expert as this is my first big leap from a small pond to something better in my roomed gardens

Kevin,
The answer would be found in your filtration system, aeration levels, and water temperature and then in sub components of your pond such as feeding habits, pond floor circulation/cleanliness (bottom drains or jets), etc.
You definitely do not have too many fish for your volume of water, but aeration levels would be something of a possible concern with only circulating the pond once every two hours. If you are in a cold climate, I would not be as concerned for the health of the fish and the water ecosystem, but if you are in a warm climate as I am in San Antonio, Texas you definitely need to rotate your pond closer to once an hour and use aeration systems to push oxygen down to the bottom of your 7′ deep pond and keep the water cooler.
With a great filter system sized properly like an Ultima2 20,000gallon filter and a Matala 150watt UV unit, your water quality should be great for your 40 Koi without issue. At my personal pond at home, I have this systems with 40 Koi in a much larger volume pond and have crystal clear water year around and can count the pebbles on the floor of my pond 7′ deep. I do though circulate my water hourly and have a Hakko 60LPM aeration system to support the ecosystem.
I would watch your nitrate levels periodically and make sure you are doing routine water changes to flush some of the toxins and hormone releases out of the pond…we recommend 10%/month water change and if at all possible, to be pulled from the floor of the pond or through a vacuuming process to help pull the worst things out.

Thanks for your comments and we hope you love having a LARGE Koi Pond…

I think I will take a two-year approach to this. Over the winter, I will come up with a plan for pumping and filtration and look at flow in the pond. In the spring I will do a thorough cleaning of the pond with the vacuum and add in the skimmer box. I will continue running aeration through the summer. I managed to pick up a 28W Laguna UV Clarifier and will put this in-line with the system. I will build the back-flushes into my weekend routine. I have clear-line hoses for back-flushing.

I will add the permanent guests in the spring and delay the idea of temporary guests a year.